BD Marine Cadet Admission key terms

The upward force a fluid exerts on an object placed in it, equal to the weight of the fluid the object displaces. It explains why heavy steel ships float.

The mass of a substance per unit volume, usually written in kilograms per cubic metre. Objects less dense than water float; denser objects sink.

The weight or volume of water pushed aside by a floating hull. A ship's displacement figure states how much water it moves, which equals its total weight.

The vertical distance from the waterline to the lowest point of a vessel's keel. It shows how much water depth the ship needs to float safely.

The height of a ship's deck edge above the water surface. A larger freeboard gives more reserve buoyancy and better protection against waves.

The ratio of a substance's density to the density of water. A value below one means the substance floats; above one means it sinks.

Force acting per unit area on a surface, measured in pascals. In liquids it increases with depth because of the weight of fluid above.

The force per unit area exerted by the weight of air in the atmosphere. At sea level it is about 101 kilopascals, and it falls with altitude.

An instrument that measures atmospheric pressure. Mariners watch it because a rapid fall often warns of approaching stormy weather.

Three rules describing how forces change motion: objects stay at rest or in uniform motion unless pushed, force equals mass times acceleration, and every action has an equal opposite reaction.

The rate at which velocity changes with time, measured in metres per second squared. It occurs whenever speed or direction changes.

Speed in a stated direction, such as 12 metres per second due north. Unlike speed alone, it includes direction and is therefore a vector quantity.

A scalar has size only, such as mass or temperature. A vector has both size and direction, such as force, velocity or displacement.

The turning effect of a force, calculated as force multiplied by perpendicular distance from the pivot. It explains how levers and ship cranes work.

A push or pull that can change an object's speed, direction or shape, measured in newtons. Weight is a force caused by gravity.

The bending of light as it passes from one transparent medium into another of different density, for example from air into water. It affects how submerged objects appear.

The bouncing back of light or sound when it strikes a surface. Smooth surfaces reflect light in a predictable direction, which is how mirrors work.

The amount of a substance containing 6.02 x 10^23 particles, called Avogadro's number. Chemists use it to count atoms and molecules in convenient batches.

The mass of one mole of a substance in grams per mole. It is found by adding the atomic masses of all atoms in the formula.

The number of particles in one mole of a substance, approximately 6.02 x 10^23. It links the microscopic world of atoms to measurable laboratory quantities.

The average mass of an element's atoms compared with one twelfth the mass of a carbon-12 atom. It is the number usually shown on a periodic table.

One of two or more forms of the same element whose atoms have equal protons but different numbers of neutrons, giving different masses.

The combining power of an element, showing how many bonds one atom can form. Hydrogen has a valency of one, oxygen of two.

The strong attraction between oppositely charged ions formed when one atom transfers electrons to another. It typically holds metals and non-metals together in salts.

A bond formed when two atoms share one or more pairs of electrons. It commonly joins non-metal atoms, as in water and carbon dioxide.

A scale from 0 to 14 measuring how acidic or alkaline a solution is. Values below 7 are acidic, 7 is neutral and above 7 is alkaline.

An acid releases hydrogen ions in water and turns blue litmus red. A base neutralises acid; an alkali is a soluble base that turns red litmus blue.

The reaction in which an acid and a base cancel each other's properties, producing a salt and usually water. It releases heat.

A reaction in which a substance loses electrons or gains oxygen. Rusting of iron and burning of fuel are everyday examples.

A reaction in which a substance gains electrons or loses oxygen. Oxidation and reduction always occur together in a redox reaction.

The gradual breakdown of a metal through chemical reaction with its surroundings, such as iron rusting in damp air. Protective paint or galvanising slows it.

The use of an electric current to drive a chemical change in an electrolyte, splitting compounds into their elements. It is used to protect ships with sacrificial anodes.

The direction of one point from another, measured clockwise in degrees from north. A bearing of 090 degrees means due east.

The angular distance of a place north or south of the equator, measured in degrees from 0 at the equator to 90 at the poles.

The angular distance of a place east or west of the prime meridian, measured in degrees from 0 to 180. It fixes position with latitude.

In a right-angled triangle, the square of the longest side equals the sum of the squares of the other two sides. It is used to find distances and courses.

The branch of mathematics dealing with the ratios between the sides and angles of triangles. Sine, cosine and tangent help solve navigation and bearing problems.

Three measures of the centre of a data set. The mean is the average, the median is the middle value in order, and the mode is the most frequent value.

A number between 0 and 1 expressing how likely an event is. It equals the number of favourable outcomes divided by the total number of equally likely outcomes.

The United Nations agency that sets global standards for shipping safety, pollution prevention and seafarer training. Its conventions are adopted by member states.

The international treaty setting minimum standards for training, certification and watchkeeping of seafarers. It governs the qualifications a cadet must eventually earn.

The main international convention for preventing pollution from ships, covering oil, chemicals, sewage, garbage and air emissions. It sets discharge limits at sea.

The international convention on safety of life at sea, covering construction, fire protection, life-saving appliances and radio equipment. It is the core safety treaty for ships.

A buoyant garment that keeps a person's head above water, worn during emergencies and abandon-ship drills. It must be fitted and checked before sailing.

The ability to distinguish colours, essential for reading navigation lights and charts. Candidates are tested with plates such as the Ishihara test.

The sharpness of eyesight, usually measured on a Snellen chart. Seafarers must meet minimum standards with or without corrective lenses.

A screening measure calculated as weight in kilograms divided by height in metres squared. It gives a rough indication of whether weight is healthy.

The ability to carry out daily tasks and emergency duties with alertness and without undue fatigue. It includes stamina, strength, flexibility and cardiovascular health.

A check of how well a person detects sounds at various pitches and volumes. Adequate hearing is required to hear alarms and verbal commands at sea.

A word that has the same or nearly the same meaning as another word, such as begin and commence. Synonym questions test vocabulary range.

A word with the opposite meaning to another word, such as arrive and depart. Antonym questions test precision in vocabulary.

The grammar rule that a verb must match its subject in number and person. A singular subject takes a singular verb, as in the ship sails.

The form of a verb that shows when an action happens, such as past, present or future. Correct tense keeps the time sequence of a sentence clear.

Voice shows whether the subject performs or receives the action. In the active voice the crew lowered the boat; in the passive voice the boat was lowered.